As a configuration for imparting redundancy to a communication route to realize an improvement in the reliability of a network, a ring type network (a ring network) in which two or more communication apparatuses are connected in a ring shape is widely adopted. In a ring type network, when a loop occurs, the bandwidth of a transmission line can be completely consumed by the infinite circulation of a broadcast frame. Other communication thereby cannot be performed. Therefore, it is necessary to prevent the infinite circulation of frames. Therefore, usually, a method of logically disconnecting (hereinafter referred to as blocking) a single port on the ring network is used. In order to prevent division of the communication route when a failure occurs on a ring network, there have been devised several methods for securing a communicable route by switching the blocked port from a port set as the blocked port to a port in which the failure occurs.
For example, with ERP (Ethernet (registered trademark) Ring Protection), which is the ring protection standard for the Ethernet (registered trademark) ring in Non Patent Literature 1, the ring networks from which a multi-ring network is configured have one port that represents one of the node apparatuses (a Ring Protection Link owner: hereinafter referred to as RPL owner) that is blocked to logically disconnect a ring on the blocked port side and avoid occurrence of a loop during normal time. When a failure occurs, a node apparatus that detects the failure blocks the port in which the failure is detected and transmits an R-APS (Ring-Automatic Protection Switching) frame, which is a control frame for failure notification, from another port. When the RPL owner receives the control frame, the RPL owner unblocks the blocked port in order to perform route switching.
In the multi-ring network of the ERP in Non Patent Literature 1, when two ring networks are connected, one ring network is set as a major ring having a closed loop shape and the other ring network is set as a sub ring having an open loop shape. The major ring means a ring network that performs failure management for a transmission line shared by a plurality of ring networks (hereinafter, shared link). The cub ring means a ring network that does not perform failure management. If the major ring and the sub ring are not set in this way, both of the rings performs failure management on the shared link when shared link failure occurs. That is, when a shared link failure occurs, ports previously blocked by the RPL owner in both the ring networks are unblocked and a loop extending across both the ring networks occurs. Therefore, in a multi-ring network, one major ring and one or more sub rings are established with respect to the ring networks connected by the shared link. The major ring and the sub rings each carry out protection independently for failures other than a failure in the shared link. When failures occur in the local rings, the unblocking of blocked ports in the local rings is performed. However, when a shared link failure occurs, only the major ring performs failure management and carries out a protection operation by performing route switching to thereby avoid occurrence of a loop.